This invention relates to a carrier for an image developer, to an electrostatic latent image developer, to an image forming apparatus by electrophotography, electrostatic recording, electrostatic printing, etc., and to an image developing method by electrophotography, electrostatic recording, electrostatic printing, etc.
Conventionally a cascade development method as disclosed in U.S. Pat. No. 2,618,552 and a magnetic brush development method as disclosed in U.S. Pat. No. 2,874,063 are known as methods for developing latent electrostatic images to visible images by use of toner. In any of these development methods, a dry two-component developer is employed.
Such a dry two-component developer is composed of relatively large carrier particles and fine toner particles which are triboelectrically held on the surface of the relatively large carrier particles by the electric force generated by the friction between the carrier particles and toner particles. When such a dry two-component developer is brought into contact with latent electrostatic images, the toner particles are attracted to the latent electrostatic images, with the bonding force between the carrier particles and the toner particles being overcome by the attracting force of the electric field formed by the latent electrostatic images for bringing the toner particles towards the electrostatic images, so that the toner particles are deposited on the latent electrostatic images, whereby the latent electrostatic images are developed to visible toner images. For the above-mentioned development, it is necessary that the toner particles be provided with accurate chargeability and charge quantity so as to be selectively attracted to the desired image area formed on a photoconductor.
In conventional developers, during the process of making a number of copies, there takes place a so-called “spent phenomenon” that a toner film is formed on the surface of the carrier particles as a result of collision between the toner particles and the carrier particles and collision between such developer particles and mechanical portions of a development unit, so that the charging performance of the carrier particles is decreased. This will result in deposition of the toner particles on the background of the images and of lowering of the copy quality. When the filming phenomenon excessively develops, there occurs the case where the developer must be exchanged with fresh one in its entirety, which will lead to an increase in the copy making costs.
To cope with the spent phenomenon, various methods have been proposed in which surfaces of carrier particles are coated with a resin having low surface energy. These methods, however, are not fully satisfactory. For example, a carrier coated with a styrene resin, a methacrylate copolymer resin or a styrene resin, which has a relatively high critical surface tension, still causes a spent phenomenon upon repeated use, though the charging characteristics thereof are good. A polytetrafluoroethylene resin, which has a low critical surface tension, can improve the spent carrier problem. However, since a polytetrafluoroethylene resin is located on the most negative side of the turboelectric series, it is ill-suited for use in a developer in which the toner is to be negatively charged. Because of its low surface energy, a silicone resin is also proposed as a coating resin for carriers with a view toward the prevention of the spent-carrier problem (Japanese Examined Patent Publication No. S44-27879 and Japanese Laid Open Patent Publication No. S50-2543). While the use of a silicone resin can prevent toner deposition onto carrier surfaces, it has a problem because the mechanical strengths thereof such as wear resistance and impact resistance are not sufficient. Thus, as upon repeated collision between the toner particles and the carrier particles and collision between such developer particles and mechanical portions of a development unit, the silicone resin coating is gradually worn so that the carrier core is exposed on the surface the carrier. As a consequence, the turboelectric characteristics of the carrier becomes so unstable that the image quality produced is lowered. Additionally, carriers having a resin coating have a large resistivity and are apt to cause the so-called edge effect (a phenomenon that image density of a center part of a large solid image is lighter than that of an edge part thereof) in developed images, resulting in deterioration of the reproducibility of solid images and half-tone images.
In an attempt to solve the above problems, there is a proposal to incorporate an electroconductive material into a resin layer of carrier particles so as to reduce the electric resistivity thereof. The carrier particles imparted with a suitable degree of the electrical conductivity can serve to provide developing electrodes so that the development of an electrostatic latent image can be conducted while maintaining close contact between the electrode and the latent image. By this expedience, not only line images but also large solid images can be reproduced with good fidelity. As the electroconductive material, the use of carbon has been proposed. However, the conventional carrier having a carbon-containing resin coating has a problem that carrier deposition is not effectively prevented when the particle size of the carrier is small.
As for developers, the use of a small size toner will greatly improve the reproducibility of dot images but, in this case, occurrence of background stains and reduction of color density are caused due to the fact that the small size toner has a large surface area to be charged. To improve charging efficiency of a small size toner, the use of a small size carrier has been proposed (Japanese Laid Open Publication No. H06-332237 and Japanese Patent Nos. 2703917 and 2769894).
The use of a small size carrier will give the following merits. Because of a large surface area, every toner can be sufficiently charged by friction so that the formation of a low charging amount toner or a reversed charged toner can be minimized. Thus, background stains and toner dispersion or blurs of a dot image can be reduced so that the dot image reproducibility is improved. Further, it is possible to reduce average charging amount of the toner. As a consequence, a high image density is obtainable. The use of a small size carrier can thus compensate demerits of a small size toner and is effective for obtaining desired properties of the small size toner.
The known small size carriers, however, have the following problems. In a developing stage for developing an electrostatic image on a photoconductor drum, a carrier is magnetically held by a developing cylinder so that the carrier is not transferred to the photoconductor drum. When the particle size of the carrier is small, however, the degree of magnetization per unit weight is so small that the carrier is easily released from the magnetic field of the developing cylinder and is trapped by the photoconductor drum (phenomenon of carrier deposition or carrier attraction). Such carrier deposition occurs more frequently as the particle size of the carrier becomes small. As the carrier deposition proceeds, the amount of the carrier in the developing zone decreases. This will result in failure to sufficiently charge the toner so that background steins of images and scattering of toner particles from the developing device may occur.